The long range objective of this research is to characterize the molecular mechanisms involved in hematopoietic progenitor cell (HPC) mobilization. The use of HPC to reconstitute hematopoiesis following myeloablative therapy has significantly improved the clinical outcome in patients with a variety of malignancies. Recently, mobilized peripheral blood HPC instead of bone marrow-derived HPC have been used because of their reduce engraftment times, relative ease of collection, and possibly reduced risk of graft-versus-host disease. Currently to mobilize HPC from the bone marrow to blood are well tolerate but not universally effective and are often associated with co-mobilization of neoplastic cells. A better understanding of the mechanisms that regulate HPC mobilization may lead to the design of novel mobilization strategies that overcome these problems. We recently showed that mobilization of HPC in response to cyclophosphamide (CY) or interleukin-8 but not fit-3 ligand is markedly impaired in granulocyte colony-stimulating factor receptor (G-CSFR) deficient mice. These surprising results suggested that G-CSFR signals in hematopoietic- or bone marrow stromal-cells play an important and previously unexpected role in HPC migration. This proposal is designed to characterize these these G- CSFR-dependent mechanisms of HPC mobilization. The following specific aims are proposed. 1. We will characterize in detail the mobilization response in G-CSFR deficient mice to CY, interleukin-12 (IL-12), or stem cell factor (SCF).. HPC mobilization in G-CSFR deficient mice in response to SCF or IL-12 will be analyzed. To explore mechanisms for the mobilization defect in G- CSFR deficient mice, the phenotype of hematopoietic cells, in particular HPC , in the bone marrow of wild-type versus G-CSFR deficient mice after CY treatment will be compared. 2. We will identify the cell type responsible for G-CSFR dependent mobilization. Preliminary studies of HPC mobilization in G-CSFR deficient radiation chimeras suggest that a functional G-CSFR on mature hematopoietic cells but not on HPC or stromal cells is required for CY- induced mobilization. The first objective of this specific aim is to confirm these surprising results and to determine whether primitive HPC are mobilized in a similar fashion. The second objective of this specific aim is to characterize G-CSF induced mobilization in these radiation chimeras. 3. We will define the role of neutrophils in G-CSFR dependent mobilization. A neutropenic mouse line will be generated by driving expression of the attenuated diphtheria toxin A subunit in myeloid cells using murine cathepsin G regulatory sequences. CY- and G-CSF induced HPC mobilization will be characterized in these mice. 4. We will define the regions of the G-CSFR that are required for HPC mobilization. CY- and G-CSF-induced mobilization will be characterized in two recently generated targeted "knock-in" mutations of the G-CSFR. The role of STAT-3 in the generation of the HPC mobilization signal by the G- CSFR will be examined.